Organic electroluminescence element

ABSTRACT

An organic electroluminescence element comprising: an anode layer, a cathode layer, and an organic luminescence layer therebetween, the organic luminescence layer having a carbazole derivative with a glass-transition temperature of 110° C. or higher, and a phosphorescent dopant. This structure makes it possible to provide an organic electroluminescence element which can make use of the triplet exciton state of the carbazole derivative even at room temperature and which has a practical life and superior heat-resistance.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/806,597filed Jun. 1, 2007, which is a continuation of application Ser. No.11/245,092, filed Oct. 7, 2005, now U.S. Pat. No. 7,226,546, which is acontinuation of application Ser. No. 10/683,435 filed Oct. 14, 2003, nowU.S. Pat. No. 6,979,414, which is a continuation of application Ser. No.09/816,415 filed Mar. 26, 2001, now U.S. Pat. No. 6,660,410, whichclaims the benefit of Japanese Patent Application No. 2000-087622 filedMar. 27, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescence element(which may be referred to as an organic EL element hereinafter). Morespecifically, the present invention relates to an organic EL elementusing a triplet exciton of an organic luminescence material (hostmaterial).

2. Description of the Related Art

Hitherto, organic EL elements wherein an organic luminescence layer isarranged between electrodes have been eagerly researched and developedfor the following reasons and the like.

(1) Since these elements are completely solid, they are easy to handleand produce.

(2) Since they can emit light by themselves, no light emitting membersare necessary.

(3) Since they can be clearly watched, they are suitable for display.

(4) They permit full color display easily.

The luminescence mechanism of such organic EL elements generally makesuse of a luminescence phenomenon, which is energy conversion phenomenoncaused when a fluorescent molecule in a singlet excited state (which maybe referred to a S1 state) in an organic luminescence medium istransited to a ground state radially.

A fluorescent molecule in a triplet excited state (which may be referredto a T1 state) in an organic luminescence medium can be supposed.However, radiative transition to a ground state is forbidden; therefore,such a molecule is gradually transited from the triplet excited state tosome other state by non-radiative transition. As a result, nofluorescence is emitted but thermal energy is radiated.

Here, singlet and triplet mean multiplicity of energy decided bycombination of total spin angular momentum and total orbital angularmomentum of a fluorescent molecule. Specifically, a singlet excitedstate is defined as an energy state in the case that a single electronis transited from a ground state, where no unpaired electrons arepresent, to a higher energy level without changing the spin state of theelectron. A triplet excited state is defined as an energy state in thecase that a single electron is transited to a higher energy level whilethe spin state of the electron is made reverse.

Needless to say, luminescence in a triplet excited state defined asabove can be observed if the luminescence is caused at a very lowtemperature, for example, at a liquefaction temperature of liquidnitrogen (−196° C.). However, this temperature is not a practicaltemperature, and the amount of the luminescence is only a little.

By the way, the total efficiency of luminescence from any conventionalorganic EL element is related to recombination efficiency (φrec) ofinjected charged carries (electrons and holes), and the probability(φrad) that generated excitons cause radiative transition. Therefore,the total efficiency (φel) of luminescence from the organic EL elementcan be represented by the following equation:

φel=φrec×0.25φrad

The coefficient (0.25) of φrad in the equation is decided from thematter that the probability that singlet excitons are generated isregarded as ¼. Therefore, even if recombination and radiativeattenuation of excitons are caused with a probability coefficient of 1,the theoretical upper limit of luminescence efficiency of the organic ELelement is 25%.

As described above, in any conventional organic EL element, tripletexcitons cannot be substantially used and only singlet excitons causeradiative transition. Thus, a problem that the upper limit of theluminescence efficiency is low arises.

Thus, literature 1 “Jpn. J. Appl. Phys., 38 (1999) L1502” discloses thateven at room temperature, triplet excitons (triplet excited state) of anorganic luminescence material (host material) are used to transferenergy from the triplet excitons to a phosphorescent dopant, so as togenerate a fluorescent luminescence phenomenon. More specifically, theliterature 1 reports that a fluorescent luminescence phenomenon iscaused in an organic EL element comprising an organic luminescence layercomposed of 4,4-N,N-dicarbazolylbiphenyl represented by the followingformula (6) and an Ir complex, which is a phosphorescent dopant.

However, the half-life of the organic EL element described in theliterature 1 is below 150 hours, and the usefulness of the organic ELelement is insufficient.

Thus, the inventor made eager investigations. As a result, the followinghas been found: the glass-transition temperature of4,4-N,N-dicarbazolylbiphenyl is as low as less than 110° C.; therefore,if the biphenyl is combined with an Ir complex, crystallization iseasily caused in the organic luminescence layer comprising thecombination to make the life of an organic EL element short.

Incidentally, in the present situation, a demand that theheat-resistance of organic EL elements for cars should be made higherhas been increasing in light of environment inside cars in summer.

Thus, an object of the present invention is to provide an organic ELelement which makes it possible to use triplet excitons of an organicluminescence material (host material) even at room temperature to emitfluorescence (including phosphorescence); has a practical life span; andhas a superior heat-resistance.

SUMMARY OF THE INVENTION

[1] According to the present invention, provided is an organic ELelement comprising:

an anode layer,

a cathode layer, and

an organic luminescence layer therebetween, the organic luminescencelayer having a carbazole derivative with a glass-transition temperatureof 110° C. or higher, and a phosphorescent dopant. Thus, theabove-mentioned problems can be solved.

This organic EL element makes it possible to use the triplet excitonstate of the organic luminescence material even at room temperature.Moreover, this element has a practical life, for example, a half-time of300 hours or more, and has superior heat-resistance. Thus, this elementcan be sufficiently used as an organic EL element for car.

[2] In the organic EL element of the present invention, it is preferredthat the carbazole derivative is at least one of compounds representedby the following general formulae (1) to (4):

wherein Ar¹ is a substituted or non-substituted aryl group having 6 to50 nucleus carbon atoms; Ar² to Ar⁷ are each independently a substitutedor non-substituted aryl or arylene group having 6 to 50 nucleus carbonatoms; Ar² and Ar³, Ar⁴ and Ar⁵, or Ar⁶ and Ar⁷ may be connected to eachother through a single bond or through O, S or substituted ornon-substituted alkylene as a connecting group; and each of repetitionnumbers m and n is an integer of 0 to 3,

wherein R¹ to R⁶ are each independently a hydrogen or halogen atom, analkyl, aralkyl, aryl, cycloalkyl, fluoroalkyl, amino, nitro, cyano,hydroxy, or alkoxy group; R⁷ and R⁸ are each independently a hydrogenatom, an alkyl, aralkyl, aryl, or cycloalkyl group; X¹ and X² are eachindependently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkylgroup; Y is a single bond, an alkyl, alkylene, cycloalkyl, aryl, oraralkyl chain; a repetition number p is an integer of 1 to 3.

wherein Ar⁸ to Ar¹¹ are each independently an aryl group having 6 to 50nucleus carbon atoms which may be substituted with an alkyl, alkoxy oraryl group; Ar⁸ and Ar⁹, or Ar¹⁰ and Ar¹¹ may be connected to each otherthrough a single bond or through O, S or substituted or non-substitutedalkylene as a connecting group; and R⁹ is an alkyl or alkoxy group, or asubstituted or non-substituted aryl group having 6 to 18 nucleus carbonatoms.

wherein Z is a trivalent nitrogen atom or an aromatic group; Ar¹² toAr¹⁴ are each independently a group represented by the following generalformula (5) or an aryl group having 6 to 50 nucleus carbon atoms; and atleast two of Ar¹² to Ar¹⁴ are groups represented by the followinggeneral formula (5):

wherein R¹⁰ to R²¹ are each independently an aryl group having 6 to 50nucleus carbon atoms which may be substituted with an alkyl, alkoxygroup having 1 to 6 carbon atoms, or a phenyl group; and groups adjacentto each other may form a cyclic structure; and a repetition number q isan integer of 0 to 3.

The organic EL element wherein this carbazole derivative is used as ahost material in the organic luminescence layer makes it possible to usethe triplet exciton state more effectively, and has a practical lifespan.

[3] In the organic EL element of the present invention, it is preferredthat the carbazole derivative has at least two carbazole skeletons.

This carbazole derivative has a large triplet energy to make it possibleto use the triplet exciton state more effectively even at roomtemperature (20° C.) and has a practical life span.

[4] In the organic EL element of the present invention, it is preferredthat the relationship of E1>E2 is satisfied in which E1 represents thetriplet energy of the carbazole derivative and E2 represents the tripletenergy of the phosphorescent dopant.

This structure makes it possible to transfer the triplet energy of thecarbazole derivative surely to the phosphorescent dopant, and to emitfluorescence using the triplet energy even at room temperature (20° C.)

[5] In the organic EL element of the present invention, it is preferredthat the triplet energy (E1) of the carbazole derivative is a value of21,000 cm⁻¹ or more.

A triplet energy of 21,000 cm⁻¹ corresponds to a light wavelength of 488nm. On the contrary, various phosphorescent dopants generally have atriplet energy which is equal to or less than the energy which 488 nmlight has. Therefore, by using the carbazole derivative having such alarge triplet energy as above, various phosphorescent dopants can beused.

Thus, by selecting an appropriate kind of the phosphorescent dopant forthe carbazole derivative having such a large triplet energy as above,luminescence in green, yellow, orange, vermilion, red and the like caneasily be obtained.

[6] In the organic EL element of the present invention, it is preferredthat the carbazole derivative has a cyclic structure whose tripletenergy is a value of 21,000 cm⁻¹ or more, and the cyclic structurecontains an aromatic ring, a hetero ring, or combination thereof.

This carbazole derivative makes it possible to transfer the tripletenergy of the carbazole derivative more effectively to thephosphorescent dopant. Specifically, if the carbazole derivative has acyclic structure having a triplet energy of less than 21,000 cm⁻¹, thetriplet energy is transferred to this cyclic structure so that thetriplet energy transferred to the phosphorescent dopant may be reduced.

[7] In the organic EL element of the present invention, it is preferredthat the phosphorescent dopant is a metal complex comprising at leastone metal selected from the group consisting of Ir (iridium), Ru(ruthenium), Pd (palladium), Pt (platinum), Os (osmium) and Re(rhenium).

This structure makes it possible to transfer energy effectively from thetriplet exciton of the carbazole derivative as a host material to themetal complex as the phosphorescent dopant.

[8] In the organic EL element of the present invention, it is preferredthat at least one ligand of the metal complex has at least one skeletonselected from the group consisting of phenylpyridine, bipyridyl andphenanthroline skeletons.

The bulky and electron-withdrawing skeleton(s) contained in the moleculemakes it possible to transfer energy effectively from the tripletexciton of the carbazole derivative to the metal complex.

[9] In the organic EL element of the present invention, it is preferredthat a blend amount of the phosphorescent dopant is 0.1 to 30 parts byweight per 100 parts of the carbazole derivative.

This structure makes it possible to mix the phosphorescent dopant withthe carbazole derivative uniformly, and transfer energy effectively fromthe triplet exciton of the carbazole derivative to the phosphorescentdopant.

[10] In the organic EL element of the present invention, it is preferredthat a hole barrier layer, an electron injection layer, or combinationthereof is arranged between the anode layer and the cathode layer, andthe hole barrier layer and the electron injection layer comprise analkali metal.

This structure makes it possible to drive the organic EL element at alower voltage, and make the life span of the element longer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view illustrating a basic structure of an organic ELelement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, embodiments of the organic EL elements of thepresent invention will be described. FIG. 1 is a sectional view of anorganic EL element 102, and illustrates a structure wherein an anodelayer 10, an organic luminescence layer 14 and a cathode layer 16 aresuccessively deposited on a substrate 18.

The following will chiefly describe a carbazole derivative (hostmaterial) and a phosphorescent dopant that constitute the organicluminescence layer 14, which is a characteristic element in the presentembodiment. Thus, the structure and the production process of the otherelements, for example, the anode layer 10 and the cathode layer 16 willbriefly be described. For elements which are not referred to, structuresand production processes which are generally known in the field oforganic EL elements can be adopted.

1. Carbazole Derivative (1) Kind 1

An embodiment of the present invention is characterized in that in theorganic luminescence layer, a carbazole derivative with aglass-transition temperature of 110° C. or higher is used as a hostmaterial.

This is because if the host material is a carbazole derivative, thetriplet exciton state of the carbazole derivative can be effectivelyused even at room temperature (20° C.) by combining the derivative witha phosphorescent dopant that will be described later. Specifically, aluminescence phenomenon can be caused by transferring energy effectivelyfrom the triplet state in the carbazole derivative to the phosphorescentdopant.

Such a carbazole derivative is preferably a carbazole derivative havingat least two carbazole skeletons. This is because the glass-transitiontemperature and triplet energy that will be described later can easilybe adjusted and the derivative can easily be mixed with thephosphorescent dopant.

The reason why the carbazole derivative having a glass-transitiontemperature of 110° C. or higher is used is as follows. If theglass-transition temperature is below 110° C., any combination thereofwith the phosphorescent dopant is very easily crystallized so that thelife becomes short. If the glass-transition temperature is below 110°C., a short circuit is easily caused in a short time when an electriccurrent is passed through the derivative at a high temperature. Thus,the environment where the organic element EL is used is excessivelyrestricted.

Therefore, the glass-transition temperature of the carbazole derivativeis more preferably from 115 to 170° C., and still more preferably from120 to 150° C.

The reason why the glass-transition temperature of the carbazolederivative is more preferably 170° C. or lower is that the kinds ofcarbazole derivatives having a glass-transition over 170° C. areexcessively restricted and the handling of the derivatives becomesdifficult because of a drop in their deposition ability.

A Differential Scanning calorimeter (DSC) is used to make it possible toobtain the glass-transition temperature (Tg) of the carbazole derivativeas a temperature of a change in the specific heat obtained when thederivative is heated at a temperature-rising rate of, for example, 10°C./minute in a nitrogen-circulating state.

(2) Kind 2

In the embodiment of the present invention, it is preferred that therelationship of E1>E2 is satisfied in which E1 represents the tripletenergy of the carbazole derivative in the organic luminescence layer andE2 represents the triplet energy of the phosphorescent dopant therein.

By combining the carbazole derivative with the phosphorescent dopantwith this relationship, the triplet exciton state of the carbazolederivative can surely be used even at room temperature. Specificallyother words, a luminescence phenomenon can be caused by transferringenergy certainly from the triplet state in the carbazole derivative tothe phosphorescent dopant.

It is also preferred that the triplet energy (E1) of the carbazolederivative is set to a value of 21,000 cm⁻¹ or more.

Specifically, the triplet energy, 21,000 cm⁻¹ corresponds to a lightwavelength of 488 nm. In general, various phosphorescent dopants have atriplet energy which is equal to or less than the energy which 488 nmlight has. Therefore, one or more selected from various phosphorescentdopants can be combined with the carbazole derivative.

Thus, by selecting the kind of the phosphorescent dopant appropriately,luminescence in green, yellow, orange, vermilion, red and the like canbe obtained.

Moreover, by setting the triplet energy (E1) of the carbazole derivativeto a value of 22,500 cm⁻¹ or more, luminescence in blue also can beobtained easily.

Preferably, the carbazole derivative has a cyclic structure whosetriplet energy is a value of 21,000 cm⁻¹ or more and the cyclicstructure contains an aromatic ring, a hetero ring, or combination onethereof.

If the carbazole derivative has such a cyclic structure, the tripletexciton state of the carbazole derivative can be effectively used evenat room temperature by combining the carbazole derivative with thephosphorescent dopant. That is, by causing the carbazole derivative tohave, for example, a cyclic structure wherein 9-arylcarbazole isconnected to a bivalent or trivalent group consisting an aromatic ring,the triplet energy can be set to 22,500 cm⁻¹ or less. Therefore, if thecarbazole derivative has such a cyclic structure, the frequency that thetriplet energy of 21,000 cm, originating from the carbazole group, istransferred in the molecule becomes small. Thus, the triplet energywhich is transferred to the phosphorescent dopant becomes relativelylarge.

(3) Kind 3

It is preferred to use, as the above-mentioned carbazole derivative,carbazole derivatives represented by the general formulae (1) to (4)alone or in combination of two or more.

In the general formulae (1) to (4) representing preferred carbazolederivatives, examples of preferred aryl groups having 5 to 50 nucleusatoms include phenyl, naphthyl, anthracenyl, phenanthryl, pyrenyl,coronenyl, biphenyl, terphenyl, pyrrolyl, furanyl, thiophenyl,benzothiophenyl, oxadiazolyl, diphenylanthracenyl, indolyl, carbazolyl,pyridyl, and benzoquinolyl, and the like.

Examples of preferred arylene groups having 5 to 50 nucleus atomsinclude phenylene, naphthylene, anthracenylene, phenanthrylene,pyrenylene, coronenylene, biphenylene, terphenylene, pyrrolylene,furanylene, thiophenylene, benzothiophenylene, oxadiazolylene,diphenylanthracenylene, indolylene, carbazolylene, pyridylene, andbenzoquinolylene, and the like.

The aromatic group having 6 to 50 carbon atoms may be substituted withone or more substituents. Preferred examples of the substituent includealkyl groups having 1 to 6 carbon atoms (such as methyl, ethyl,i-propyl, n-propyl, s-butyl, t-butyl, pentyl, hexyl, cyclopentyl andcyclohexyl groups); alkoxy groups having 1-6 carbon atoms (such asmethoxy, ethoxy, i-propoxy, n-propoxy, s-butoxy, t-butoxy, pentoxy,hexyloxy, cyclopentoxy and cyclohexyloxy groups); aryl groups having 5to 50 nucleus atoms; amino groups substituted with an aryl group having5 to 50 nucleus atoms; ester groups having an aryl group having 5 to 50nucleus atoms; ester groups having an alkyl group having 1 to 6 carbonatoms; a cyano group; a nitro group; halogen atoms. The above-mentionedsubstituent may be substituted with a carbazolyl group.

Moreover, as shown in the general foemulae (1) and (3) described below,the carbazole in the present invention is interpreted as a moiety formedby connecting at least two aryl groups, each of which is connected to anitrogen atom, to each other through a single bond or a connectinggroups. In this case, preferred examples of the connecting groupsinclude O, S, and substituted or non-substituted alkylene and silylene,and the like.

Here, preferred specific examples of the carbazole derivativerepresented by the formula (1) include a group of compounds illustratedas the following chemical formulae (7) to (24).

Preferred specific examples of the carbazole derivative represented bythe formula (2) include a group of compounds illustrated as thefollowing chemical formulae (25) to (29). In the formula, a methyl groupmay be abbreviated to Me.

Preferred specific examples of the carbazole derivative represented bythe formula (3) include a group of compounds illustrated as thefollowing chemical formulae (30) to (41).

Preferred specific examples of the carbazole derivative represented bythe formula (4) include a group of compounds illustrated as thefollowing chemical formulae (42) to (49).

Furthermore, specific examples of the carbazole derivative having astructure other than the structures represented by the general formulae(1) to (4) include a group of compounds illustrated as the followingchemical formulae (50) to (59).

wherein a repetition number is an integer of 3 to 20.

2. Phosphorescent Dopant (1) Kind

{circle around (1)} Metal Complex

The phosphorescent dopant is preferably a metal complex comprising atleast one metal selected from the group consisting of Ir, Ru, Pd, Pt, Osand Re.

This is because if the phosphorescent dopant is any one of these metalcomplexes, energy can be effectively transferred from triplet excitonsof the carbazole derivative as a host material to the phosphorescentdopant.

More specific examples of the phosphorescent dopant are metal complexessuch as tris(2-phenylpyridine)iridium, tris(2-phenylpyridine)ruthenium,tris(2-phenylpyridine)palladium, bis(2-phenylpyridine)platinum,tris(2-phenylpyridine)osmium, tris(2-phenylpyridine)rhenium, octaethylplatinum porphyrin, octaphenyl platinum porphyrin, octaethyl palladiumporphyrin, and octaphenyl palladium porphyrin. In order to transferenergy more effectively to emit fluorescence, more preferred are metalcomplexes comprising Ir, for example, tris(2-phenylpyridine)iridiumrepresented by the following formula (60):

{circle around (2)} Ligand of the Metal Complex

At least one ligand of the metal complex preferably has at least oneskeleton selected from the group consisting of phenylpyridine, bipyridyland phenanthroline skeletons.

This is because by at least one of these electron withdrawing skeletonscontained in the molecule, energy can be effectively transferred fromthe triplet excitons of the carbazole derivative to the metal complex.

Particularly, in the phosphorescent dopant, the ligand preferably has aphenylpyridine skeleton among these skeletons, such astris(2-phenylpyridine)iridium.

(2) Added Amount

A blend amount of the phosphorescent dopant is preferably 0.1 to 30parts by weight per 100 parts by weight of the blended carbazolederivative (host material).

The reasons for this are as follows. If the blend amount of thephosphorescent dopant is below 0.1 part by weight, the effect based onthe blend is not exhibited so that energy may not be effectivelytransferred from triplet excitons of the carbazole derivative. On theother hand, if the blend amount of the phosphorescent dopant is over 30parts by weight, the phosphorescent dopant is not easily blended withthe carbazole derivative uniformly so that luminescence brightness maybe scattered.

Therefore, the blend amount of the phosphorescent dopant is morepreferably 0.5 to 20 parts by weight, and is still more preferably 1 to15 parts by weight.

3. Other Organic Layers in the Organic Luminescence Medium (1) HoleInjection Layer

It is preferred to deposit a hole injection layer having a thickness of5 nm to 5 μm. The deposition of such a hole injection layer makes itpossible to inject holes satisfactorily into the organic luminescencelayer, give a high luminescence brightness, and attain driving at a lowvoltage.

It is also preferred to use, in the hole injection layer in the organicluminescence medium, a compound having a hole mobility of 1×10⁻⁶cm²/V·second or more and an ionization energy of 5.5 eV or less. Thehole mobility is measured when a voltage of 1×10⁴ to 1×10⁶ V/cm isapplied to the hole injection layer.

Specific examples of the constituent material of the hole injectionlayer are organic compounds such as porphyrin compounds, aromatictertiary amine compounds, styrylamine compounds, aromatic dimethylidynecompounds, and condensed aromatic ring compounds, for example,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviated to NPD) and4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviated to MTDATA).

As the constituent material of the hole injection layer, an inorganiccompound such as p-type Si or p-type SiC is preferably used.

It is also preferred to arrange an organic semiconductor layer having anelectric conductivity of 1×10⁻¹⁰ S/cm or more between the hole injectionlayer and the anode layer, or between the hole injection layer and theorganic luminescence layer. The arrangement of the organic semiconductorlayer makes the injection of holes into the organic luminescence layermore satisfactory.

(2) Electron Injection Layer

It is preferred to deposit an electron injection layer having athickness of 5 nm to 5 μm. The deposition of such an electron injectionlayer makes it possible to inject electrons satisfactorily into theorganic luminescence layer, give a high luminescence brightness, andattain driving at a low voltage.

It is also preferred to use, in the electron injection layer, a compoundhaving an electron mobility of 1×10⁻⁶ cm²/V·second or more and anionization energy over 5.5 eV. The electron mobility is measured when avoltage of 1×10⁴ to 1×10⁶ V/cm is applied to the electron injectionlayer.

Specific examples of the constituent material of the electron injectionlayer are metal complexes of 8-hydroxyquinoline (Al chelate: Alq),derivatives thereof, and oxadiazole derivatives.

If an alkali metal is incorporated into the electron injection layer inthe same way as into a hole barrier layer that will be described later,the organic EL element can be driven at a notably low voltage and thelife thereof can be made longer.

(3) Hole Barrier Layer

It is preferred to arrange a hole barrier layer having a thickness of 5nm to 5 μm between the organic luminescence layer and the cathode. Thearrangement of the hole barrier layer makes it possible to improvecapability of confining holes in the organic luminescence layer, give ahigh luminescence brightness, and attain driving at a low voltage.

Examples of the constituent material of the hole barrier layer include2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and2,9-diethyl-4,7-diphenyl-1,10-phenanthroline, and the like. Morepreferably, an alkali metal such as Li or Cs is further added thereto.

The combination of the alkali metal with the hole barrier layerconstituting material in the hole barrier layer makes it possible todrive the organic EL element at a notably low voltage and make the lifethereof longer.

When the alkali metal is incorporated, the amount thereof is preferably0.01 to 30% by weight. (if the total amount of the hole barrier layer is100% by weight)

If the amount of the alkali metal is below 0.01% by weight, the effectof the addition thereof may be not exhibited. On the other hand, if theamount is over 30% by weight, the dispersion of the alkali metal becomesuneven so that luminescence brightness may be scattered.

Therefore, the amount of the alkali metal is more preferably 0.05 to 20%by weight, and more still preferably 0.1 to 15% by weight.

4. Electrode (1) Anode Layer

The anode layer corresponds to a lower electrode or an oppositeelectrode, dependently on the structure of the organic EL displaydevice. The anode layer is preferably made of a metal, an alloy or anelectrically conductive compound having a large work function (forexample, 4.0 eV or more), or a mixture thereof. Specifically, it ispreferred to use one or a combination of two or more electrode materialsselected from indium tin oxide (ITO), indium zinc oxide (IZO), copperiodide (CuI), tin oxide (SnO₂), zinc oxide (ZnO), gold, platinum,palladium and the like.

By using these electrode materials, the anode layer having a uniformthickness can be made using a method making deposition in a dry statepossible, such as vacuum evaporation, sputtering, ion plating, electronbeam evaporation, CVD (Chemical Vapor Deposition), MOCVD (Metal OxideChemical Vapor Deposition), or plasma CVD.

When EL luminescence is taken out from the anode layer, it is necessaryto make the anode layer to a transparent electrode. In this case, it ispreferred to use an electrically conductive material such as ITO, IZO,CuI, SnO₂ or ZnO to set the transmissivity of EL luminescence to a valueof 70% or more.

The thickness of the anode layer is not particularly limited. Thethickness is preferably a value of 10 to 1,000 nm, and more preferably avalue of 10 to 200 nm.

If the thickness of the anode layer is set to a value within such arange, uniform thickness distribution can be obtained and thetransmissivity of EL luminescence can be made to 70% or more. Moreover,the sheet resistivity of the anode layer can be made to a value of 1000Ω/□ or less, and preferably 100 Ω/□ or less.

It is also preferred that light is emitted from an arbitrary pixel inthe luminescence face by depositing the anode layer (lower electrode),the organic luminescence medium, and the cathode layer (oppositeelectrode) successively and making the lower electrode and the oppositeelectrode into an XY matrix pattern. By making the anode and so on intothis manner, various data can easily be displayed in the organic ELelement.

(2) Cathode Layer

The cathode layer also corresponds to a lower electrode or an oppositeelectrode, dependently on the structure of the organic EL displaydevice. The cathode layer is preferably made of a metal, an alloy or anelectrically conductive compound having a small work function (forexample, less than 4.0 eV), or a mixture thereof.

Specifically, it is preferred to use any one or a combination of two ormore electrode materials selected from sodium, sodium-potassium alloy,cesium, magnesium, lithium, magnesium-silver alloy, aluminum, aluminumoxide, aluminum-lithium alloy, indium, a rare earth metal, a mixture ofan organic luminescence medium material and these metals, a mixture ofan electron injection layer material and these metals, and the like.

The thickness of the cathode layer is not particularly limited. Thethickness is preferably a value of 10 to 1,000 nm, and more preferably avalue of 10 to 200 nm.

Furthermore, when EL luminescence is taken out from the cathode layer,it is necessary to make the cathode layer to a transparent electrode. Inthis case, it is preferred to set the transmissivity of EL luminescenceto a value of 70% or more.

The cathode layer is preferably formed by a method making deposition ina dry state possible, such as vacuum evaporation or sputtering, in thesame way as for the anode layer.

5. Supporting Substrate

The supporting substrate in the organic EL element is preferably asubstrate which has superior mechanical strength and small permeabilityof moisture or oxygen. Specific examples thereof include glass plates,metal plates, ceramic plates and plastic plates (such as polycarbonateresin, acrylic resin, vinyl chloride resin, polyethylene terephthalateresin, polyimide resin, polyester resin, epoxy resin, phenol resin,silicone resin, and fluorine resin plates) and the like.

To avoid invasion of moisture into the organic EL element, it ispreferred to form an inorganic film or apply a fluorine resin onto thesupporting substrate made of such a material as above to conductmoisture-proof treatment or hydrophobic treatment.

Particularly to avoid invasion of moisture into the organic luminescencemedium, it is preferred to make the water content in the supportingsubstrate and the gas transmissivity thereof small. Specifically, it ispreferred to set the water content in the supporting substrate to0.0001% or less by weight and set the gas transmissivity thereof to1×10⁻¹³ cc·cm/cm²·sec. cmHg or less.

EXAMPLE Example 1 (Production of an Organic EL Element)

{circle around (1)} Washing

A glass substrate (made by Geomatic company) 25 mm in width, 75 mm inlength and 1.1 mm in thickness, with ITO transparent electrodes, wassubjected to ultrasonic washing in isopropyl alcohol for 5 minutes andsubjected to UV ozone washing for 30 minutes.

{circle around (2)} Formation of a Hole Injection Layer

The washed glass substrate with the ITO transparent electrodes was setto a substrate holder in a vacuum evaporation device, and thenN,N-bis(N,N-diphenyl-4-aminophenyl)-N,N-diphenyl-4,4′-diamino-1,1′-biphenyl(abbreviated to TPD232 hereinafter) was vapor-deposited on the substrateat a vacuum degree of 665×10⁻⁷ Pa and a vapor-deposition rate of 0.1 to0.3 nm/sec., so as to form a first hole injection layer (which also hada function as a hole transport layer) having a thickness of 60 nm.

4,4-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviated to NPDhereinafter) was vapor-deposited on the TPD232 film at a vacuum degreeof 665×10⁻⁷ Pa and a vapor-deposition rate of 0.1 to 0.3 nm/sec., so asto form a second hole injection layer (which also had a function as ahole transport layer) having a thickness of 20 nm.

{circle around (3)} Formation of an Organic Luminescence Layer

Next, the same vacuum evaporation device was used to vapor-deposit acarbazole compound (Tg: 110° C. or higher) represented by the formula(9) on the NPD film formed in the previous step at a vacuum degree of665×10⁻⁷ Pa and a vapor-deposition rate of 0.1 to 0.3 nm/sec., so as toform an organic luminescence layer having a thickness of 30 nm.

In this case, at the same time of the vapor deposition of the carbazolederivative compound represented by the formula (9),tris(2-phenylpyridine)iridium was vapor-deposited as a phosphorescentdopant. In this vapor co-deposition, the vapor-deposition rate of thephosphorescent dopant was adjusted in the manner that the ratio of theamount of the blended phosphorescent dopant to the total amount of theorganic luminescence layer would be 7% by weight. (if the total amountof the organic luminescence layer is 100% by weight)

{circle around (4)} Formation of a Hole Barrier Layer

Next, the same vacuum evaporation device was used to vapor-deposit2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (abbreviated to BCPhereinafter) on the organic luminescence layer formed in the previousstep at a vacuum degree of 665×10⁻⁷ Pa and a vapor-deposition rate of0.1 to 0.3 nm/sec., so as to form an organic luminescence layer having athickness of 10 nm.

{circle around (5)} Formation of an Electron Injection Layer

Next, the same vacuum evaporation device was used to deposit atris(8-quinolinol)aluminum film (abbreviated to an Alq film hereinafter)on the hole barrier layer formed in the previous step at a vacuum degreeof 665×10⁻⁷ Pa and a vapor-deposition rate of 0.1 to 0.3 nm/sec., so asto form an electron injection layer.

At this time, Li (Li source made by Saesu getter company) and Alq weresubjected to vapor co-deposition in the manner that the molar ratiobetween them would be 1:1, so that the electron injection layer was madeto an Alq/Li film having a thickness of 20 nm.

{circle around (6)} Formation of a Cathode

Next, the same vacuum evaporation device was used to vapor-deposit metalAl on the electron injection layer formed in the previous step at avacuum degree of 665×10⁻⁷ Pa and a vapor-deposition rate of 0.5 to 1.0nm/sec., so as to form a cathode having a thickness of 150 nm.

{circle around (7)} Sealing Step

The resultant organic EL element was put into a dry box into whichnitrogen was introduced. Furthermore, its luminescence face was coatedwith blue glass and the periphery thereof was treated with acation-setting adhesive TB3102 (made by Three Bond Co., Ltd.) to performsealing.

In this way, the organic EL element of Example 1 was prepared.

(Evaluation of the Organic EL Element)

A DC voltage of 6 V was applied between the anode and the cathode in theresultant organic EL element, so that green luminescence having aluminescence brightness of 1,200 cd/m² and a luminescence efficiency of40 cd/A was obtained.

The organic EL element was driven at a low voltage. The initialbrightness thereof was set to 500 cd/m². In this way, a life span testwas performed. As a result, the half-time, which is a driving time untilthe initial brightness becomes half, was 500 hours. This half-time ispractical.

A current-sending test as a heat-resistance test was performed at 85° C.Even after current was sent for 200 hours, green luminescence havingsufficient luminescence brightness was obtained.

The obtained results are shown in Table 1.

Example 2

An organic EL element was produced and evaluated in the same way as inExample 1 except that at the time of vapor-depositing BCP for the holebarrier layer in Example 1, metal Li, which is an alkali metal, and BCPwere subjected to vapor co-deposition at a molar ratio of 1:1.

As a result, even by application of a DC voltage of 5 V, greenluminescence having a luminescence brightness of 1,300 cd/m² and aluminescence efficiency of 37 cd/A was obtained.

The organic EL element was driven at a low voltage. The initialbrightness thereof was set to 500 cd/m². In this way, a life span testwas performed, so that the half-time was 700 hours.

A current-sending test as a heat-resistance test was performed at 85° C.Even after current was sent for 200 hours, green luminescence havingsufficient luminescence brightness was obtained.

Example 3

An organic EL element was produced and evaluated in the same way as inExample 1 except that at the time of vapor-depositing BCP for the holebarrier layer in Example 1, metal Cs, which is an alkali metal, and BCPwere subjected to vapor co-deposition at a molar ratio of 1:1.

As a result, even by application of a DC voltage of 4.5 V, greenluminescence having a luminescence brightness of 1,200 cd/m² and aluminescence efficiency of 40 cd/A was obtained.

The organic EL element was driven at a low voltage. The initialbrightness thereof was set to 500 cd/m². In this way, a life span testwas performed, so that the half-time was 800 hours.

A current-sending test as a heat-resistance test was performed at 85° C.Even after current was sent for 200 hours, green luminescence havingsufficient luminescence brightness was obtained.

Comparative Example 1

An organic EL element was produced and evaluated in the same way as inExample 1 except that instead of the carbazole compound represented bythe formula (9), a carbazole compound (Tg: less than 110° C.)represented by the formula (6) was used in the organic luminescencelayer.

As a result, by application of a DC voltage of 6 V, green luminescencehaving a luminescence brightness of 1,100 cd/m² and a luminescenceefficiency of 38 cd/A was obtained.

The organic EL element was driven at a low voltage. The initialbrightness thereof was set to 500 cd/m². In this way, a life span testwas performed. However, the half-time was as short as 150 hours. Thishalf-time is not practically allowable.

A current-sending test as a heat-resistance test was performed at a hightemperature of 85° C. A short circuit was caused after 100 hours. Theorganic EL element was unable to be lighted.

This demonstrated that the organic EL element of Comparative Example 1had poor heat-resistance and was unable to be used for car.

TABLE 1 Organic 85° C. luminescence Hole Luminescence Luminescencecurrent- layer barrier Voltage brightness efficiency LuminescenceHalf-time sending Tg (° C.) layer (V) (nit) (cd/A) color (hours) test(hours) Example 1 Formula Only 6 1200 40 Green 500 >200 (9) > 110 BCPExample 2 Formula BCP/Li = 5 1300 37 Green 700 >200 (9) > 110 1/1Example 3 Formula BCP/Cs = 4.5 1200 40 Green 800 >200 (9) > 110 1/1Comparative Formula Only 6 1100 38 Green 150 <200 Example 1 (6) > 110BCP

Examples 4 to 9

An organic EL element was produced and evaluated in the same way as inExample 1 except that instead of the compound represented by the formula(9) of Example 1, each of compounds represented by the formulae (10),(19), (26), (30), (43) and (55) was used as a carbazole compound. Theobtained results are shown in Table 2.

TABLE 2 Organic 85° C. luminescence Hole Luminescence Luminescencecurrent- layer barrier Voltage brightness efficiency LuminescenceHalf-time sending Tg (° C.) layer (V) (nit) (cd/A) color (hours) test(hours) Example 4 Formula Only 6 1050 35 Green 450 >200 (10) > 120 BCPExample 5 Formula Only 6 600 18 Green 350 >200 (19) > 110 BCP Example 6Formula Only 6 450 10 Green 300 >200 (26) > 110 BCP Example 7 FormulaOnly 6 890 28 Green 400 >200 (30) > 110 BCP Example 8 Formula Only 6 92030 Green 550 >200 (43) > 140 BCP Example 9 Formula Only 6 350 10 Green400 >200 (55) > 110 BCP

INDUSTRIAL APPLICABILITY

According to the organic EL element of the present invention, itsorganic luminescence medium comprises a carbazole derivative with aglass-transition temperature of 110° C. or higher, and a phosphorescentdopant, so that the triplet exciton state of the carbazole derivativecan be used even at room temperature and this element has a practicallife and superior heat-resistance.

1. An organic electroluminescence element comprising: an anode layer, a cathode layer, and an organic luminescence layer therebetween, the organic luminescence layer having a carbazole derivative with a glass-transition temperature of 110° C. or higher, and a phosphorescent dopant, wherein the carbazole derivative is represented by the following general formula (2):

wherein R¹ to R⁶ are each independently a hydrogen or halogen atom, an alkyl, aralkyl, aryl, cycloalkyl, fluoroalkyl, amino, nitro, cyano, hydroxy, or alkoxy group; R⁷ and R⁸ are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; X¹ and X² are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; Y is a single bond, an alkylene, cycloalkylene, arylene, or aralkylene chain; a repetition number p is an integer of
 1. 2-3. (canceled)
 4. The organic electroluminescence element according to claim 1, wherein relationship of E1>E2 is satisfied in which E1 represents triplet energy of the carbazole derivative and E2 represents triplet energy of the phosphorescent dopant.
 5. The organic electroluminescence element according to claim 1, wherein triplet energy E1 of the carbazole derivative is a value of 21,000 cm⁻¹ or more.
 6. The organic electroluminescence element according to claim 1, wherein the carbazole derivative has cyclic structure whose triplet energy is a value of 21,000 cm⁻¹ or more, and the cyclic structure contains an aromatic ring, a hetero ring, or combination thereof.
 7. The organic electroluminescence element according to claim 1, wherein the phosphorescent dopant is a metal complex comprising at least one metal selected from the group consisting of Ir, Ru, Pd, Pt, Os and Re.
 8. The organic electroluminescence element according to claim 7, wherein at least one ligand of the metal complex has at least one skeleton selected from the group consisting of phenylpyridine, bipyridyl and phenanthroline skeletons.
 9. The organic electroluminescence element according to claim 1, wherein a blend amount of the phosphorescent dopant is 0.1 to 30 parts by weight per 100 parts of the carbazole derivative.
 10. The organic electroluminescence element according to claim 1, wherein a hole barrier layer, an electron injection layer, or combination thereof is arranged between the anode layer and the cathode layer, and the hole barrier layer and the electron injection layer comprise an alkali metal. 